Merc 40 EFI 4-stroke vapor lock - The Hull Truth

Craig West · 05-31-2008, 08:33 PM

I work mainly on Yanaha and Honda, but have been having some difficulty with a little 40 EFI Mercury that I was hoping someone might be able to help me with. The 40 merc had a surging problem directly linked to fluctuating injector fuel prressure. I replaced the HP fuel pump and inline HP fuel filter. This sorted out the problem. My customer took it out and ran it, but when he fired it up to come home, it started surging again and quit. He was able to get home at idle. Next thing I changed was the Pressure regulator which had a pretty clogged filter I have tested it since but can still duplicate the customers complaint by running it hard then stopping and letting it heatsoak for 20 minutes. When I restart and give it gas, it runs for a couple seconds then surges and eventually stalls. I Squeezed the primer bulb but no change. Tested fuel pump (LP) vacuum and pressure was within spec. Removed cooling lines from HP fuel cooler and LP fuel pump and found adequate water flow....to tell tail. I noticed that Mercury had introduced the fuel coolers because of this problen. Please let me know if I am overlooking something simple or if you have heard of something like this before.
Many thanks.
Craig

A1 Outboard · 06-02-2008, 05:57 AM

First you have to determine if the boat's fuel system is delivering the proper amount of solid liquid fuel with no air bubbles.

Temporarily hook up a piece of clear fuel line to the fuel pump inlet fitting, and Tee in a vacuum gauge. Run the motor at all speeds and look for air bubbles in the clear line or for a vacuum restriction of over 4"Hg. You can also watch for vapor lock by letting it heatsoak and see if the liquid fuel in the line starts to vaporize.

Because the pressure regulator had a clogged filter, the whole EFI system may have to come apart to a thorough cleaning and send the injectors out to be flowtested.

Also do not overlook a temp sender that is reading incorrectly which will tell the ECU to deliver the wrong fuel mixture to the enigne.

timeoff2fish · 06-10-2008, 06:46 PM

I've got a 60 HP Merc 4-stroke doing exactly the same thing. Any progress?